Earth’s Energy Balance

• 100 units of solar radiation hits the top of the atmosphere

• Surface absorbs 51 solar units (49 reflected & absorbed)

• Surface absorbs another 96 units from the warm sky!• Atmosphere emits 96 units down (warm) but 64 up

(cold)• Surface has to get rid of 147 units: 117 by radiation,

23 by evaporated water, only 7 by rising hot air

Atmosphere is Warmed from

Below

The Job of the Atmosphere

is to let the energy out!

“Piles up” in tropics

“Escapes” near poles and aloft

The movement of the air (and oceans) allows energy to be transported to its “escape zones!”

Energy In

• North-south contrast• Land-sea contrast• Ice and snow• Deserts vs forests

Annual Mean

Energy Out• Given by esT4

(which T?)• Combined surface and

atmosphere effects• Decreases with

latitude• Maxima over

subtropical highs (clear air neither absorbs or emits much)

• Minima over tropical continents (cold high clouds)

• Very strong maxima over deserts (hot surface, clear atmosphere)

Annual Mean

Energy In minus Energy Out

• Incoming solar minus outgoing longwave

• Must be balanced by horizontal transport of energy by atmosphere and oceans!

Earth's Energy BalanceEarth's annual

energy balance between solar insolation and terrestrial infrared radiation is global but not local

The global balance is maintained by transferring excess heat from the equatorial region toward the poles

It Takes a Lot of Energy

to Evaporate Water!

Energy Balance of Earth’s Surface

Radiation Turbulence

shortwavesolar

radiationlongwave(infrared)radiation

rising warm

air

evaporatedwater

H LE

Energy from the Surface to the Air

• Energy absorbed at the surface warms the air

• Some of this energy is transferred in rising warm “thermals”

• But more of it is “hidden” in water vapor

Rising Warm Air (H)

Evaporated Water (LE)

A sunlight beam that strikes at an angle is spread across a greater surface area, and is a less intense heat source than a beam impinging directly.

Seasons & Solar Intensity

Solstice & Equinox

• At solstice, one pole is light & one is dark (24/7)• At equinox, tilt provides exactly

12 hours of night and 12 hours of day everywhere

Midnight Sun

The region north of the Arctic Circle experiences a period of 24 hour sunlight in summer, where the Earth's surface does not rotate out of solar exposure

March 20, Sept 22

June 21

Dec 21

NH summer

Equinox

NH winter

Daily Solar at Top of Atmosphere

• 75º N in June gets more sun than the Equator!

• Compare N-S changes by seasons

• Very little tropical seasonality

Regional Seasonal CyclesRegional differences in

temperature, from annual or daily, are influenced by geography, such as latitude, altitude, and nearby water or ocean currents, as well as heat generated in urban areas

San Francisco is downwind of the Pacific Ocean

Richmond, VA is downwind of North America!

Local Solar Changes Northern

hemisphere sunrises are in the southeast during winter, but in the northeast in summer

Summer noon time sun is also higher above the horizon than the winter sun

Landscape Solar Response

South facing slopes receive greater insolation, providing energy to melt snow sooner and evaporate more soil moisture. North and south slope terrain exposure often lead to differences in plant types and abundance.

Temperature Lags RadiationEarth's surface

temperature is a balance between incoming solar radiation and outgoing terrestrial radiation.

Peak temperature lags after peak insolation because surface continues to warm until infrared radiation exceeds insolation.